1. Field of the Invention
The present invention relates to a test apparatus and method for determining at least one characteristic of a test specimen, and in one application, relates to a test apparatus and method for determining a transformation temperature of a shape memory metal alloy. An exemplary application is a test apparatus and method for determining the martensite-to-austenite transformation temperatures of nickel titanium alloys.
2. Description of the Related Art
ASTM Method F2082 is a test method for the determination of transformation temperatures of nickel titanium shape memory metal alloys by bend and free recovery. This test method involves cooling a test specimen to its nominally fully martensite phase, deforming the specimen, and heating the specimen to its fully austenite phase. During heating, the motion of specimen is measured and is plotted versus specimen temperature for determining the martensite-to-austenite transformation temperatures.
A test apparatus for carrying out this method is contemplated in ASTM Standard F2082, and generally includes a fixture in which a specimen is held, the fixture submerged in a cooling bath. The specimen is deformed in the bath, and placed on the fixture which holds the specimen so as to not interfere with the free recovery of the specimen on heating. The bath is heated, and a thermocouple is used to measure the temperature of the bath. A transducer is used to detect the change in shape of the specimen as the specimen straightens, and displacement and temperature are measured and plotted versus one another to determine physical characteristics of the specimen, such as the martensite-to-austenite transformation temperature.
One known test device for implementing ASTM Standard F2082 is the Shape Track™ device, available from Confirmd LLC of San Carlos, Calif., and is similar to the test apparatus contemplated by ASTM Standard F2082. This device is based on the use of a linear variable differential transducer (LVDT) which is placed in contact with the test specimen to measure displacement. However, it is thought that contact of the transducer with the specimen could adversely affect the accuracy of the test data. Also, liquid nitrogen is used to cool the bath, which can be both hazardous and costly. Further, the bath temperature is not accurately controlled per the ASTM specification, as a hotplate is used, which cannot provide uniform and consistent temperature changes. Finally, this apparatus can only test one specimen at a time.
What is needed is a test apparatus and method for determining transformation temperatures of shape memory metal alloys which is an improvement over the foregoing.